This invention relates to a fixing device having a fixing belt and an image forming apparatus having the fixing belt for image formation used for an electrophotographic copying machine, printer, facsimile, and so on.
Recently, the world has been increasing the demand of higher image qualities in the electrophotographic image forming apparatus such as copying machines, printers, facsimile equipment, and the like. Fixing devices of some image forming apparatus have been demanded to make images glossier or controlled.
Conventionally, thermal roller fixing devices have been well known as fixing devices. A thermal roller fixing device is equipped with a heating rubber roller which has an elastic layer around it and is controlled to keep it at a preset temperature and a pressing rubber roller which is in contact with the heating rubber roller and has an elastic layer around it. A recording sheet having an unfixed toner image on it is sandwiched, heated and transferred by these heating and pressing rubber rollers. During the movement of the sheet, the toner image on the sheet is fixed.
However, in such a thermal roller fixing device, the heating rubber roller has a great heat capacity and takes a long warm-up time. Further, the inner side of the elastic layer is heated too much. This shortens the service life of the heating rubber roller.
To solve the above problems, belt-type fixing devices using an endless belt member (a fixing belt) have been also known. The fixing belt of the belt-type fixing device is engaged with a fixing roller and a heating roller having a heating means to heat the fixing belt. The belt-type fixing device is further equipped with a pressing roller which provided opposite to the fixing roller with the fixing belt between them. A recording sheet having an unfixed toner image on it is sandwiched, heated and transferred by the fixing belt and the pressing roller. During the movement of the sheet, the toner image on the sheet is fixed.
Simultaneously, a single image forming apparatus has been demanded to form images of different degrees of glossiness such as color photo images, OHP images, and mono-chromatic character images.
In an image fixing method using a heating roller, the fixing performance such as a fixing ability (adhesion of toner to recording sheets) and a degree of glossiness is singly determined by a pair of fixing and pressing rollers. Therefore, when the pressure is varied to change the degree of glossiness, the fixing nip width or the fixing time is also varied. This affects the other fixing performance.
This is because the fixing function is concentrated to a single point and consequently limits the exact variation of degree of glossiness.
Contrarily to the image fixing method using a heating roller, some technologies below have been proposed to enable the belt fixing method to vary the degree of glossiness.
One technology consists of steps of detecting the degree of glossiness of a manuscript, varying the airflow rate of the fan to cool the fixing device and a point at which a transfer medium separates from the fixing belt according to the detected information of glossiness, changing the fixing temperature and time thereby, and thus getting a desired degree of glossiness of an image. This technology is disclosed, for example, by Japanese Non-examined Patent Publication H5-333643.
However, by this technology using a cooling means to change the airflow rate, an exact glossiness control cannot be expected because the cooling ability is affected by the environmental temperature change. Further, when the above-described separation point is changed, the sheet delivery path varies and this affects the sheet delivery characteristic.
Another technology consists of steps of changing the fixing speed and the fixing temperature according to a Glossy image mode, an OHP image mode, or a Non-glossy mode by a preset program and getting desired glossiness. This technology is disclosed, for example, by Japanese Non-examined Patent Publication 2001-282034.
However, this technology reduces the productivity of images of the image forming apparatus when the fixing speed is reduced. Similarly, when a temperature change is required, it takes a lot of time to change the temperature. This also reduces the image productivity of the image forming apparatus.
A still another technology gets a desired degree of image glossiness by calculating a desired temperature of the heating roller from an entered glossiness mode, a detected temperature of the pressing roller, a load nip time, etc. and controlling the heat source for the heating roller to this desired roller temperature. This technology is disclosed, for example, by Japanese Non-examined Patent Publication 2003-149987.
However, this technology reduces the productivity of images of the image forming apparatus as it takes a lot of time to change the temperature of the heating roller.